I cannot seem to get my grass to fill out. I have big patches where it won't grow. The grass is also very thick stems and rough.....not like the thin pluss grass of the neighbors. I have tried seeding, fertz, etc. I have thatched it and aerated. I have noticed that the sod is very old and several inches thick down till you get to dirt. Any advice?

My neighbor is currently dealing with this issue. He has a very small lawn so he took out all the lawn down to good soil, added more soil then reseeded the whole lawn. If your lawn is bigger with just bad spots, you could likely do the same in a bad areas.